Cyanuric chloride is produced on a large scale by chlorination of hydrogen cyanide with the formation of cyanogen chloride and trimerisation of the cyanogen chloride to form cyanuric chloride—see Ullmann's Encyclopedia of Industrial Chemistry Vol. A8, 5th ed. (1987), 196-197. The trimerisation is carried out in the vapour phase at a temperature of above 200° C., in particular in the range of about 300 to 450° C., on an activated carbon catalyst. During continuous operation, a temperature profile develops along the longitudinal axis of the reactor owing to the exothermicity of the trimerisation reaction; this results in the formation of a so-called hot-spot, the temperature maximum of which depends on the flow rate and rises with increasing flow rate. It is known that the deactivation of the activated carbon catalyst is influenced by the operating conditions, the flow rate and the quality of the activated carbon. The deactivation becomes apparent from the movement of the reaction zone, and with it the temperature maximum, along the longitudinal axis of the catalyst.
Owing to its becoming deactivated, the catalyst has to be exchanged periodically or otherwise activated. The economic efficiency of the cyanuric chloride process depends considerably on the service life of the catalyst, as not only the cost of the catalyst but also the cost of a plant standstill have to be taken into account. Moreover, with increasing deactivation of the catalyst, secondary products such as, for example, cyameluric chloride, are increasingly discharged and hence necessitate increased expenditure on the purification of the cyanuric chloride.
In view of the problems demonstrated, the experts have for a long time been interested in finding activated carbon catalysts which have an increased service life and/or in varying the operating conditions in such a way that the service life can be increased.
Accordingly, U.S. Pat. No. 3,312,697 discloses a process for producing cyanuric chloride using an activated carbon catalyst having a specific surface of above 1000 m2/g, in which the activated carbon catalyst was activated by a treatment with acids and/or alkalies and a downstream washing with water. As a result of the above-mentioned treatment, inorganic constituents such as oxides, hydroxides and salts of metals such as Li, Mg, Ce, Ti, V, Mn, Fe, Ni, Pt, Cu, Zn, Cd, Sn, Pb and Bi, which diminish the service life of the catalyst, are dissolved out of the activated carbon. The service life of the catalyst is further increased in this process by the addition of 0.5 to 10 wt. % chlorine and/or phosgene to the cyanogen chloride.
In the process according to U.S. Pat. No. 3,707,544, the service life is increased by mixing the trimerisation reactor with a mixture of an activated carbon and a solid diluent having little or no catalytic activity. The disadvantage of this process is that the space-time yield is diminished and the expense of disposing of the deactivated catalyst is increased, above all if the diluent is a non-combustible material.
In the process described in U.S. Pat. No. 3,867,382, an untreated activated carbon produced from coconut shells is used instead of an acid-washed activated carbon. This activated carbon has an internal surface area of 1200 to 1500 m2/g, a micropore volume of at least 0.7 cm3/g and an ash content of below 4 wt. %. Owing to the vegetable origin of the raw material used for this activated carbon, it has a low content of heavy metals and an acid wash is rendered unnecessary. It cannot be inferred from this document how the micropores are defined, i. e. whether they comprise all the internal pores, or micropores having precisely defined limiting values for the pore diameters. A considerable disadvantage of the activated carbon used in the examples is that the bulk density, and hence the quantity required based on the reactor volume, is very high and thus diminishes the economic efficiency.
In J. Beijing Inst. Chem. Technol. 20 (1993) 1, 55-58, E. Wang et al. explain that several factors, namely, the ash content, the iron content, the specific surface and the pore-size distribution, have to be taken into account when selecting the catalysts for the cyanogen chloride trimerisation. The selection of a suitable activated carbon is complicated by the fact that these factors may mutually influence one another. It is to be concluded from this document that it is advantageous to use a carbon which has as high a specific surface as possible and therefore contains numerous small pores. The latter help to enable the reaction to proceed on a relatively large number of active centres. From the diagrams of the pore-size distribution of two different activated carbons, it is suggested that the pores should have a diameter in particular of less than 2 nm. However, no information can be drawn from the document as to how the individual factors influence the service life of the catalyst in a production plant designed for continuous operation.